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The OSI Model and the TCP/IP Protocol Suite
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To discuss the idea of multiple layering in data communication and networking and the interrelationship between layers.
To discuss the OSI model and its layer architecture and to show the interface between the layers.
To briefly discuss the functions of each layer in the OSI model.
To introduce the TCP/IP protocol suite and compare its layers with the ones in the OSI model.
To show the functionality of each layer in the TCP/IP protocol with some examples.
To discuss the addressing mechanism used in some layers of the TCP/IP protocol suite for the delivery of a message from the source to the destination.
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2-1 PROTOCOL LAYERS
In Chapter 1, we discussed that a protocol is required when two entities need to communicate.Whencommunication isnotsimple,wemaydivide the complex task of communication into severallayers.Inthiscase,wemayneedseveralprotocols,oneforeachlayer. Letususeascenarioincommunicationinwhich the role of protocol layering may be better understood. We use two examples. In the first example, communication is so simple that it canoccurinonlyonelayer.
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Hierarchy Services
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Assume Maria and Ann are neighbors with a lot of common ideas.However,MariaspeaksonlySpanish,andAnnspeaks only English. Since both have learned the sign language intheirchildhood,theyenjoymeetinginacafeacoupleofdaysperweekandexchangetheir ideasusingsigns.Occasionally, theyalsouseabilingualdictionary.Communication is face tofaceandHappensinonelayerasshowninFigure2.1.
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Figure 2.1 Example 2.1
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NowassumethatAnnhastomovetoanothertownbecauseofherjob.Beforeshemoves,thetwomeetforthelasttimeinthesamecafe.Althoughbotharesad,MariasurprisesAnnwhensheopensapacketthatcontainstwosmallmachines.ThefirstmachinecanscanandtransformaletterinEnglishtoasecretcodeorviceversa.TheothermachinecanscanandtranslatealetterinSpanishtothesamesecretcodeorviceversa.Anntakesthefirstmachine;Mariakeepsthesecondone.Thetwofriendscanstillcommunicateusingthesecretcode,asshowninFigure2.2.
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Figure 2.2 Example 2.2
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2-2 THE OSI MODEL
Established in 1947, the International StandardsOrganization(ISO)isamultinationalbodydedicatedtoworldwideagreementoninternationalstandards. Almost three-fourths of countries in the world arerepresentedintheISO.AnISOstandardthatcoversallaspectsofnetworkcommunicationsistheOpen Systems Interconnection (OSI) model. It was firstintroducedinthelate1970s.
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Layered Architecture Layer-to-layer Communication Encapsulation Layers in the OSI Model Summary of OSI Layers
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ISO is the organization; OSI is the model.
Note
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Figure 2.3 The OSI model
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Figure 2.4 OSI layers
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Figure 2.5 An exchange using the OSI model
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The physical layer is responsible for moving individual bits from one
(node) to the next.
Note
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Figure 2.6 Summary of OSI Layers
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2-3 TCP/IP PROTOCOL SUITE
TheTCP/IPprotocolsuitewasdevelopedpriortothe OSI model. Therefore, the layers in the TCP/IPprotocolsuitedonotmatchexactlywiththoseintheOSImodel.TheoriginalTCP/IPprotocolsuitewas defined as four software layers built upon thehardware.Today,however,TCP/IPisthoughtofasafive-layermodelwiththelayersnamedsimilarlyto theones in theOSImodel.Figure2.7showsbothconfigurations.
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Comparison between OSI and TCP/IP Layers in the TCP/IP Suite
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Figure 2.7 Layers in the TCP/IP Protocol Suite
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Figure 2.8 TCP/IP and OSI model
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Figure 2.9 A private internet
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Figure 2.10 Communication at the physical layer
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The unit of communication at the physical layer is a bit.
Note
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Figure 2.11 Communication at the data link layer
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The unit of communication at the data link layer is a frame.
Note
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Figure 2.12 Communication at the network layer
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The unit of communication at the network layer is a datagram.
Note
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Figure 2.13 Communication at transport layer
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The unit of communication at the transport layer is a segment, user
datagram, or a packet, depending on the specific protocol used in this layer.
Note
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Figure 2.14 Communication at application layer
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The unit of communication at the application layer is a message.
Note
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2-4 ADDRESSING
Four levels of addresses are used in an internet employing theTCP/IP protocols: physical address, logical address, port address, and application-specific address.Eachaddress is related toaonelayerintheTCP/IParchitecture,asshowninFigure2.15.
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Physical Addresses Logical Addresses Port Addresses Application-Specific Addresses
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Figure 2.15 Addresses in the TCP/IP protocol suite
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InFigure2.16anodewithphysical address10 sendsa frame toanodewithphysicaladdress87.The twonodesareconnectedbyalink(aLAN).Atthedatalinklayer,thisframecontainsphysical(link)addressesintheheader.Thesearetheonlyaddressesneeded.Therestoftheheadercontainsotherinformationneededatthislevel.As the figure shows, the computer with physical address 10 is thesender,and thecomputerwithphysicaladdress87 is the receiver.Thedatalinklayeratthesenderreceivesdatafromanupperlayer.Itencapsulates thedata ina frame.Theframeispropagatedthrough theLAN.Eachstationwithaphysicaladdressother than87drops the frame because the destination address in the frame does notmatchitsownphysicaladdress.Theintendeddestinationcomputer,however,findsamatchbetweenthedestinationaddressintheframeanditsownphysicaladdress.
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Figure 2.16 Example 2.3: physical addresses
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AswewillseeinChapter3,mostlocalareanetworksusea48-bit (6-byte)physicaladdresswrittenas12hexadecimaldigits;everybyte (2hexadecimaldigits) isseparatedbyacolon,asshownbelow:
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Figure2.17 showsapart of an internetwith two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case,eachcomputer isconnectedtoonlyonelinkandthereforehasonlyonepairofaddresses.Eachrouter,however, isconnected to threenetworks.Soeachrouterhasthreepairsofaddresses,oneforeachconnection.Althoughitmaybeobviousthateachroutermusthaveaseparatephysicaladdressforeachconnection,itmaynotbeobvious why it needs a logical address for each connection. We discuss these issues inChapters11and12whenwediscuss routing.Thecomputerwith logicaladdressAandphysicaladdress10needs tosendapacket to thecomputerwith logicaladdressPandphysical address 95. We use letters to show the logical addresses and numbers for physical addresses, but note that both are actuallynumbers,aswewillseeinlaterchapters.
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Figure 2.17 Example 2.5: logical addresses
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The physical addresses will change from hop to hop, but the logical
addresses remain the same.
Note
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Figure 2.18 shows two computers communicating via the Internet.Thesendingcomputer is running threeprocessesat this time with port addresses a, b, and c. The receiving computer is running two processes at this time with portaddressesjandk.Processainthesendingcomputerneedsto communicate with process j in the receiving computer. Note thatalthoughbothcomputersareusing thesameapplication,FTP,forexample,theportaddressesaredifferentbecauseoneisaclientprogramandtheotherisaserverprogram,aswewillseeinChapter17.
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Figure 2.18 Example 2.6: port numbers
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The physical addresses change from hop to hop, but the logical and port addresses usually remain the same.
Note
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As we will see in future chapters, a port address is a 16-bitaddressrepresentedbyonedecimalnumberasshown.
753A16-bitportaddressrepresentedasonesinglenumber